Abstract: Heavy air pollution accompanied by widespread haze clouds occurred in January 2013 over the North China Plain (NCP). To investigate the controlling factors of the haze formation during the study period of 10-15 January 2013, the air quality modeling system RAMS-CMAQ and multiple observation data including routine weather measurements and the air pollution index (API) released by the Ministry of Environment Protection of China were applied to analyze the temporal and spatial variation features of the meteorological factors and key aerosol components. The simulation results show that a high mass burden of PM2.5, which was higher than the 120 μg m^-3 generally distributed in the Beijing-Tianjin-Tangshan region, central and south Hebei Province, and most parts of the Shandong Province. The visibility over these regions broadly ranged from 5 to 8 km. Furthermore, the mass burden of PM2.5 reached 250-300 μg m^-3 over Beijing, Tianjin, Shijiazhuang, Jinan, and their surrounding regions and exceeded 300 μg m^-3 over their urban areas. Thus, the visibility dropped to 3 km and formed severe haze clouds over these cities. Analysis of the meteorological field shows that compared with the multi-year average, the horizontal wind speed and relative humidity at the surface decreased approximately 20% and increased 10%-40%, respectively, over the Beijing-Tianjin-Tangshan region, central and south Hebei Province, and northern Shandong Province during this pollution episode. Coupled with temperature inversion, these stable atmospheric conditions were favorable for pollutant accumulation, and the extinction ability of soluble particles was significantly enhanced due to the high relative humidity. Thus, the abnormal meteorological field was a primary reason for the haze formation in January 2013 over the NCP. Sulfate, nitrate, and ammonium were the main aerosol components responsible for the haze formation in Beijing. The contribution of these three types of inorganic salts to the total extinction exceeded 50%. Additionally, the nitrate provided the first contribution to the surface extinction at more than 25%, which implies that emissions from the transportation sector, in addition related industry sectors, were major anthropogenic sources of this pollution episode in Beijing.